It has long been known that the cetane number of diesel oils can be improved either by adding a nitrogen-containing fuel additive, or by oxidation with a nitrogenous oxidizing agent. Oils in the diesel boiling point range having the proper physical characteristics such as pour point, cloud point, viscosity and volatility can be obtained by nitrating the diesel fraction in order to increase the cetane number. However, it is well known that the nitration of such oils tends to increase the Ramsbottom carbon content and to decrease to stability of the fuels by forming an insoluble sediment. The insoluble sediment produces a haze and eventually a deposit while the fuels are in storage. While many attempts to eliminate the disadvantage of poor stability characteristics have been made and solvent extraction, including alkali scrubbing, has been employed to improve stability, conventional solvent extraction has proven unsatisfactory to provide acceptable stability in the case of nitrogen-treated fuels.
Solvent extraction with certain organic solvents, such as those described in copending U.S. patent application Ser. Nos. 832,612, filed 2/24/86 (attorney's Docket No. A-4963) and 832,197, filed 2/24/86 (attorney's Docket No. A-4962) is effective to improve stability and reduce Ramsbottom carbon content, but a major portion of the cost of upgrading diesel oil by this method is incurred in the solvent extraction process. A method of improving the cetane rating of substandard diesel oils that does not require an expensive solvent extraction step to meet diesel fuel product specifications for stability and Ramsbottom carbon is particularly desirable.
It is known to enhance the cetane number of diesel oil using oxidative processes. Regarding the use of nitration to improve the cetane number of diesel oils, British Pat. No. 491,648 teaches contacting a diesel oil with a nitrating agent in order to increase its cetane number. The disclosure in British Pat. No. 491,648 is that the nitrated oil can be used alone or in a blend with untreated diesel oil. Extraction with solvents including acetone, methyl and ethyl alcohols, ethylene dichloride and aniline is described for obtaining concentrates of nitrated petroleum components. However, by contacting a diesel oil with a nitrating agent, stability is decreased and Ramsbottom carbon is increased, and these deficiencies are combined with poor process yields when the product is extracted using the solvents disclosed. While recognizing the problem of sedimentation and poor stability, this British Patent contains no suggestion as to how these problems can be reduced or eliminated.
U.S. Pat. No. 3,135,680 discloses oxidation of a sour petroleum fraction with nitrogen dioxide followed by washing with water and alkali, to desulfurize diesel oils and improve cetane. The disclosure is that the product obtained, however, tends to have an objectionable color resulting from the nitrogen dioxide treatment, and subsequent sulfuric acid treatment, vacuum distillation or clay treating is considered essential to completely remove materials formed during oxidation. Unfortunately, this reduces or eliminates the increase in cetane number. Due to its high Ramsbottom carbon content, the product of this process forms substantial coke in the still upon distillation.
A process is described in U.S. Pat. No. 3,164,546 for producing diesel fuels having improved cetane number and odor, by treating the oil with nitrogen dioxide, washing with aqueous alkali and/or solvent extraction, followed by a water wash. Solvents disclosed as suitable for the solvent extraction step are nitromethane, dimethylformamide, pyridine, acetonitrile, glycolonitrile, ethylene glycol, ethanolamine and phenol. No reference is made, however, to the important stability and Ramsbottom carbon content specifications, which are by far the most difficult product specifications to meet for a diesel fuel product when employing nitrogenous treating agents. Furthermore, extraction reduces yields of product.
U.S. Pat. No. 2,333,817 discloses oxidation of diesel oils by nitrogenous compounds followed by hexane dilution and filtering to improve cetane and prevent sediment formation. Such a product does not pass present-day industry standard for stability (although haze formation is reduced) and it does not meet Ramsbottom carbon specifications. From the exemplification it appears this technique is only applicable to cycle oils.
These methods have generally apparently employed nitration or an oxidation treatment for cetane improvement of diesel oils, but it is also known that the cetane number of diesel oils can be increased by adding various nitrated hydrocarbon derivatives to the oils, including amyl nitrate, octyl nitrate, and the nitrate ester "dopes" disclosed in British Pat. No. 491,648. Other nitrogen-containing additives for improving cetane are disclosed in U.S. Pat. No. 4,398,505. Use of these derivatives is disadvantageous since they are expensive and must be separately prepared, handled and stored.
U.S. Pat. No. 2,184,440 relates to methods of increasing the cetane number of blended diesel fuels by blending diesel fuels from different sources, one of which is a high sulfur crude, and treating the blended fuel with sodium plumbite and a large excess of elemental sulfur. Alternatively, a distillate fuel stock can be treated with sodius plumbite and an excess amount of elemental sulfur over that required to sweeten the stock, and distilled to obtain a sweetened condensate, with the bottoms being further reduced and added to an untreated diesel fuel to increase the cetane number of the blend. The reduced bottoms amounting to about 1 to 6% of the original distillate fuel stock are added in amounts of about 0.5 to 5% to diesel fuels to raise their cetane number, e.g., by 2 to 19 over untreated diesel fuel.
U.S. Pat. No. 2,104,919 provides a means to achieve a stabilized fuel by blending straight-run and cracked stocks. The disclosure of this patent states that typically fuel oils produced by blending straight run components and cracked residues have a tendency to form a carbonaceous sludge on storage, and the precipitation of sludge in such fuels containing 10 to 80% heavy cracked residues is inhibited by mildly oxidizing the straight-run component before blending, e.g., by blowing air at 250.degree.-400.degree. C. therethrough for 1/2 to 12 hours.
U.S. Pat. No. 2,317,968 discloses a diesel fuel containing substantial portions of chemically combined reactive oxygen through treatment with air, oxygen or an oxygen carrying gas, to produce by partial oxidation a petroleum fuel having a volatility greater than that of kerosene and relatively free from asphaltic and resinous components and from large proportions of aromatics, to an "oxygen factor" of 800 to 1450 by air blowing, removal of acids produced by oxidation, and blending with a diesel fuel to produce a diesel fuel blend having increased cetane. Blends of 2.5% oxidized oil to 97.5% by volume sulfur dioxide-treated petroleum distillate and of 20% by volume of oxidized oil to 80% by volume of a clean gas oil distillate are disclosed.
U.S. Pat. No. 2,365,220 discloses a method similar to that of U.S. Pat. No. 2,317,968. The disclosure is of the preparation of a diesel fuel, in which a diesel fuel of predominately paraffinic character is oxidized using air, oxygen or an oxygen carrying gas to produce an oil having an "oxygen factor" of higher than 5, acidic reaction products are removed, and the resulting oxidized stock is blended with from 2/5 to 20 times its volume of a clean diesel petroleum hydrocarbon distillate.
U.S. Pat. No. 2,521,698 discloses that the fuels produced by the methods of U.S. Pat. Nos. 2,317,968 and 2,365,220 are unstable and corrosive, and that the stability and corrosiveness of such cetane-enhanced diesel fuels can be improved with a loss of about 25% of cetane enhancement, by subjecting the oxidized diesel fuel to acid treatment with a strong acid, such as sulfuric or nitric acid, in a concentration of at least 0.1 pound per gallon of oxidized stock, followed by alkali wash and blending of the treated oxidized stock with diesel fuel. A concentrated oxygenated cetane-improving additive is also disclosed, prepared from the bottoms of the distilled oxidized diesel stock.
U.S. Pat. No. 4,280,818 discloses oxidizing a hydrocarbon oil with aqueous nitric acid in a weight ratio of about 1:0.1 to 1:10 of hydrocarbon to acid and separating the aqueous phase from the oxidized hydrocarbon phase. The disclosure is that the oxidized hydrocarbon phase can be extracted and the product obtained blended with a polar solvent to produce a fuel mixture.
U.S. Pat. No. 3,284,342 discloses oxidation of residue using a number of oxidants including nitrogen oxides, followed by a thermal treatment to reduce the sulfur content of the residue, in which both steps can be promoted with catalysts. When applied to diesel oil, this process produces substantial carbonaceous deposits in the thermal treating still, and is unsatisfactory for commercial use.
U.S. Pat. No. 3,135,680 discloses a process for refining petroleum fractions in the diesel oil boiling range to produce diesel fuel with enhanced cetane number and odor by treating the fractions with nitrogen dioxide followed by a clay treatment to remove odor.
A process of deodorizing and desulfurizing light petroleum distillates by treatment with nitrogen dioxide followed by alkali wash and water wash is disclosed in U.S. Pat. No. 3,267,027. This process is unsuitable for producing diesel fuels of acceptable stability and Ramsbottom carbon content.
U.S. Pat. No. 3,244,618 discloses a process for sweetening petroleum hydrocarbons by treating the hydrocarbon fraction with molecular oxygen in the presence of a catalytic amount of a nitrogen oxide. Application of this process to diesel fuel results in a product with inferior cetane enhancement.
U.S. Pat. No. 2,004,849 discloses the use of an oxidant, hydrogen peroxide, in combination with sulfuric acid to effect sulfur removal from hydrocarbons, without substantial loss of aromatics. However, this process is ineffective for improving the cetane of diesel fuel.
Processes for treating petroleum stocks by oxidation followed by solvent extraction have been described for various purposes.
For example, a process for producing a fuel composition by oxidizing a hydrocarbon oil with aqueous nitric acid, followed by extraction with acetone, methyl ethyl ketone, cyclohexanone, methanol, ethanol, normal propanol, isopropanol, ethyl acetate, tetrahydrofuran, dioxane, or a combination of an alcohol and a ketone, an alcohol and water, a ketone and water or a combination of alcohols is disclosed in U.S. Pat. No. 4,280,818.
Although the oxidation/extraction methods described above have met with some success in improving petroleum diesel fuels, the known approaches toward oxidation to remove a portion of the original sulfur content as gaseous sulfur oxides, and to convert a portion of the original sulfur content into sulfoxides and/or sulfones followed by extraction with appropriate solvents to achieve a desired low sulfur raffinate have not completely eliminated problems of instability and unacceptable Ramsbottom carbon for diesel fuels, and have the disadvantage of an expensive solvent extraction step, resulting in low yields.
The methods described above when applied to petroleum stocks for use as diesel fuels basically have the disadvantages that (a) oxidative desulfurization methods involving nitrogenous oxidizing agents often result in increased gum and sedimentation, and reduce the stability of the fuels produced, (b) the oxidatively treated fuels are not useful as diesel fuel blendstocks due to poor stability and high Ramsbottom carbon content, and (c) solvent extraction, while effective to improve stability and Ramsbottom carbon content, is expensive and of low product yields in comparison to oxidative treatment alone where the oxidized product alone can be blended to improve diesel fuels.
While many conventional methods of improving diesel cetane number by oxidation with nitrogenous oxidizing agents exist, they are generally inadequate to meet other product specifications. Particularly, diesel fuels produced by nitrogeneous oxidation and solvent extraction can in some cases meet sulfur and cetane requirements for fuels, but are unsatisfactory with respect to the important specifications of stability and Ramsbottom carbon content. Processes employing sulfuric acid or clay in conjunction with nitrogeneous oxidizing agents are ineffective to retain a high cetane rating when practiced to control Ramsbottom carbon and stability. Distillative methods are commercially unfeasible due to the presence of substantial carbonaceous deposits in the still, and when thermal treating is applied to diesel fuel to reduce sulfur content of the residue, this process also produces substantial carbonaceous deposits in the thermal treating still.
Apart from the failure of conventional oxidative cetane enhancement methods to provide diesel fuels of sufficient stability and Ramsbottom carbon content, these methods, like the oxidative desulfurization methods, employ solvents which result in poor yields, requiring unacceptably high solvent-to-oil ratios. Alternatively, the solvents used in some methods reduce or entirely eliminate the advantage of cetane enhancement obtained by oxidation.
While certain of the patents cited above refer to oxidation of diesel stocks to improve cetane, and the use of such oxidatively enhanced stocks in blends with unoxidized diesel fuels, these patents fail to recognize the importance of limiting the nitrogen content of the final blend to achieve acceptable stability or Ramsbottom carbon content in the final blend. Furthermore, there is no appreciation of the remarkable increase of cetane in the treated portion and in the final blend when nitrogen treatment and blending are controlled according to the present process.